1. Field
The present disclosure relates to techniques for modulating optical signals. More specifically, the present disclosure relates to an integrated optical device that includes an electro-absorption modulator.
2. Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, low latency and low power consumption for inter-chip and intra-chip optical interconnects or links. A key component for use in inter-chip and intra-chip optical interconnects is a modulator that can be monolithically integrated into the same silicon layer as transistors and other optical components.
Some approaches for implementing silicon-photonic modulators are based on germanium, which is an efficient absorbing material at infrared wavelengths that include a 1.5 μm band for electro-absorptive modulator applications. For example, for high-performance transistors, a silicon layer can either be strained by the addition of germanium layers or used as a direct, mobility-enhanced silicon-germanium layer.
Consequently, there is considerable interest in integrating alloys of germanium on silicon from both an optical and an electronic perspective. In existing fabrication techniques, germanium alloys are typically produced by direct heteroepitaxial integration using crystal growth at the wafer level. For this reason, germanium is available in many front-end CMOS manufacturing lines and opportunistically provides a manufacturing path for making an electro-absorptive modulator on silicon.
Several approaches have been proposed for making germanium-based modulators that are monolithically integrated on silicon. In some approaches, an silicon-on-insulator (SOI) substrate is used to provide strong confinement of light for optical-waveguide applications in conjunction with germanium integration. The use of a SOI substrate can also be used to fabricate a plurality of wavelength-division-multiplexing optical components to further increase optical communication bandwidth.
Moreover, direct heteroepitaxial integration has been reported for germanium quantum wells and silicon-germanium alloys in surface-normal geometries on bulk silicon substrates to produce quantum-confined Stark effect (QCSE) and Franz-Keldysh (FK) electro-absorption modulators. In principle, electro-absorption modulators, with their wide spectral range of modulation-bandwidth operation, may allow reduced tuning power to be used when aligning the electro-absorption modulators with other optical components in optical links. However, in practice a surface-normal device geometry often requires thick active layers (in order for the light to interact with the silicon-germanium layer). Therefore, very high voltage is typically needed during operation of these optical devices. Furthermore, these optical devices usually are not compatible with the in-plane, edge geometries that are typically used for optical-waveguide operation.
Integrating such a vertical profile on an SOI substrate remains an active area of research. For example, attempts at integrating in-plane FK and QCSE electro-absorption modulators in optical waveguides on SOI substrates are typically complicated and usually have poor performance.
Hence, what is needed is an integrated electro-absorption modulator without the above-described problems.